ch smaller diameter, concentric
with the main tube (Fig. 3, at A). The oxygen is mixed with the gas
during its escape from the inner tube, which is pierced by a number of
fine holes for the purpose, the extreme end being closed up. The
inner tube may run up to within half an inch of the point where the
cap carrying the nozzle joins the larger tube.

Fig. 2.

Fig. 3.

If it is desired to use the blow-pipe for working glass which is
already fixed in position to a support, it will be found very
advantageous to use a hooked nozzle. The nozzle shown in the sketch
is not hooked enough for this work, which requires that the flame be
directed 'backwards towards the worker. With a little practice such a
flame may be used perfectly well for blowing operations on the table,
as well as for getting at the back of fixed tubes.

To warm up the glass, the gas supply is turned full on, and enough
oxygen is allowed to pass in to clear the flame. The work is held in
front of, but not touching, the flame, until it is sufficiently hot to
bear moving into the flame itself. The, work is exposed to this flame
until, in the case of lead glass, traces of reduction begin to appear.
When this point is reached the oxygen tap is thrown wide open. I
generally regulate the pressure on the bags, so that under these
circumstances the flame is rather overfed with oxygen. This condition
is easily recognised, as follows. The flame shrinks down to a very
small compass, and the inner blue cone almost disappears; also
flashes of yellow light begin to show themselves--a thing which does
not occur when the proportions of the gases are adjusted for maximum
heating effect.

For many purposes the small dimensions of the flame render it very
convenient, and the high temperature which can be attained at exact
spots enables glass to be fused together after a certain amount of
mixing, which is an enormous advantage in fusing lead glass on to hard
glass. The lead glass should not be heated hot enough to burn, but,
short of this, the more fluid it is the better for joints between
dissimilar samples.

It will be noticed that the blow-pipe can be rotated about a vertical
axis so as to throw the flame in various directions. This is often
indispensable.

Sec. 15. In general the oxygen flame does not require to be delivered
under so high a pressure as for the production of a lime light. In
England, I presume, most experimenters will obtain their ox